Plaster of Paris casts have been in use to immobilize body members or limbs for some time. The plaster of Paris bandages have been supplemented and, to some extent, superseded by synthetic cast tapes or bandages which employ polymeric materials on a substrate. The preferred polymeric materials are water-cured or water-reactive polyurethane compositions. The polyurethane materials have largely supplanted other polymeric synthetic casting materials. These polyurethane casting materials are of the type which are disclosed in U.S. Pat. Nos. 4,376,438 and 4,411,262.
The fibrous substrate used in the synthetic casting materials is usually a polyester or fiberglass. Although knitted substrates are most common, woven substrates have also been used. The fiberglass materials offer advantages in terms of strength of the finished cast and various constructions of fiberglass fabrics have been used for the substrates for the synthetic casting tapes. The patents mentioned above disclose the use of different fiberglass materials as the substrate for casting tapes. In addition, U.S. Pat. Nos. 3,686,725, 3,787,272 and 3,882,857 disclose specific fiberglass materials, or the treatment of fiberglass yarns, to produce fiberglass substrates which are particularly suitable for use in orthopaedic casts.
U.S. Pat. No. 4,323,061 discloses a cast substrate made from a combination of glass fibers and a second fiber such as cotton, flax, rayon, wool, acrylic resin, nylon, Teflon or polyester. The purpose of the second fiber in the substrate is to hold the curable resin on the substrate.
U.S. Pat. No. 3,332,416 discloses a plaster of Paris cast bandage with a woven substrate made with a combination of elastic and inelastic fibers.
Although fiberglass has been extensively used as a substrate material in orthopaedic casts, with different reactive polymers, all of these casting materials suffer certain disadvantages. One of the major disadvantages is the conformability of the casting tape to the body of the patient. Conformability is the characteristic of the casting tape which has been defined as that property which describes the ability of the bandage or casting tape to adapt to or intimately lay down against the compound curves and protrusions of a body member. Fiberglass casting tapes are generally stiffer than casting tapes made of other fibers, and cast technicians and surgeons have some difficulty conforming the fiberglass tapes to the limbs of a patient.
Casting tapes with improved conformability combine elastic and nonelastic yarns in the tape substrate. U.S. Pat. No. 4,668,563 discloses a polyurethane casting tape made from a high modulus fiber such as fiberglass, polyaramide or polyethlene combined with an elastomeric highly extensible fiber made from natural or synthetic rubber or spandex (polyurethane).
U.S. Pat. No. 5,256,134 discloses a polyurethane casting tape containing an elastic yarn such as natural or synthetic rubber or polyurethane and an inelastic yarn formed from polypropylene, polyester, polyamide, polyethylene or cotton viscose.
A disadvantage of the conformable casting tapes mentioned above is that the elastic fibers employed had serious limitations. As discussed in U.S. Pat. No. 4,668,563, the water reactive polyurethane prepolymer may eventually swell the spandex (polyurethane) filaments causing the filaments to lose their extensibility. This limits the shelf life of conformable casting tapes made with spandex elastic filaments. Natural and synthetic rubber filaments are usually compounded with chemicals which may cause the polyurethane prepolymer to gel prematurely. This may be avoided by treating the rubber filaments with an extraction process or by treating the rubber filaments with an acid. Both of these processes are environmentally detrimental and add cost to the substrate and casting tape.